1. Field of the Invention
The invention pertains to an adjustable-speed drive mechanism for imparting and receiving rotational force, wherein said mechanism comprises a variable diameter pulley assembly and a flexible belt or chain wrapped around the axle of the pulley, that serves as a gripping surface.
2. The Prior Art
Variable diameter pulley systems have long been used in the mechanical transmission art, and are well known as inexpensive, but highly efficient means to derive infinitely variable output speeds from a constant power source, or means to cause an increase or a decrease in torque from a variable power source.
For example, in U.S. Pat. No. 4,179,946 there is disclosed a variable diameter, centrifugally responsive, tension operated pulley, which when used in combination with a substantially similar pulley and belt, provides a controlled-speed variable ratio accessory drive.
An automatic transmission for small motorized vehicles is disclosed in U.S. Pat. No. 3,747,721, and incorporates variable diameter pulleys, whereby drive ratios can easily be changed between upper and lower limits.
The June 1989 issue of Machine Design magazine presents an overview of variable speed transmissions in an article entitled "Mechanical Adjustable-Speed Drives." This magazine indicates that the efficiency of a variable diameter pulley system is usually about 95%, and that such systems provide good overload and jam protection due to favorable slips (i.e., the belts provide good overload and jam protection because the belt slips when overloaded).
However, despite the aforementioned advantages, variable diameter pulleys are known to be critically limited in relation to other types of mechanical transmissions insofar as the amount of useful power which can be handled due to the natural torque limitations of the belt and pulley combination itself.
Accordingly, a need exist in the mechanical transmission art, for a reliable method of increasing the torque handling capability of a variable diameter pulley.
With all current belt and pulley systems, contact between the belt and the pulley occurs on the side of the belt. Because of the small thickness of the belt necessary for belt flexibility, the contact area is small. In addition, the power transfer occurs by friction because the variable diameter prevents installing gear teeth or other positive engaging mechanisms on the side of the pulley. In this connection, it should be noted that some variable pulleys have ribs on the contact surfaces; however, the ribs and/or their spacing increases with increasing distance from the axis of the pulley half. Such systems can provide for a non-slip engagement, but only for very limited power, since the changing spacing on the pulley halves is incompatible with the fixed spacing on the belt.